The response characteristics of neurons in the early visual system are often studied in anesthetized animals, under conditions in which their eyes are paralyzed. Contrast sensitivity functions of neurons measured in these conditions deviate significantly from behavioral measurements of contrast sensitivity -- psychophysical measurements peak at higher spatial frequencies and exhibit stronger low-frequency suppression than their neurophysiological counterparts. One possible basis for the discrepancy is lack of consideration of the effect of abolishing eye movements in neurophysiological recordings. Microscopic eye movements are always present during natural fixation, and they have been shown to enhance high spatial frequency vision. Here, we examine how neuronal response properties determined in the absence of retinal image motion apply to natural viewing conditions. We describe an "equivalent filter" for retinal ganglion cells which combines measures of neural responses determined with an immobile stimulus and the statistics of human fixational eye movements. We show that consideration of fixational eye movements eliminates the discrepancy between psychophysical and neurophysiological measurements. For both P and M cells at all the considered visual eccentricities, neuronal sensitivity to time-varying inputs shifts towards higher spatial frequencies when the influence of fixational eye movements is taken into account. Thus, our model predicts that contrast sensitivity functions measured with paralyzed eyes significantly underestimate the actual sensitivity to high spatial frequencies present in natural viewing conditions.